Assuming you already have a full email server (either hosted by you or you are using some third party service), you will need your internal server (the home server) to be able to send emails. This is true even if you host your email server, because as i say here, you cannot self-host a real email server on your home server.
The following must be performed on your internal server.
The Gentoo default mail-mta/nullmailer package is more than enough. It should be already installed, otherwise emerge it:
emerge mail-mta/nullmailer
I assume you already have a mail domain somewhere linked to your @mydomain.com, and you have your own address as myself@mydomain.com and you created a specific mailbox called home@mydomain.com to manage any email sent from services.
All the following files need to be under /etc/nullmailer:
myself@mydomain.com
mydomain.com
mail.mydomain.com smtp port=587 user=home@mydomain.com pass=**** starttls
Try it from the command line:
(echo "Subject: test email"; echo "Hi! my nullmailer works...") | sendmail -F "Me Myself" -f myself@mydomain.com destination@otherdomain.de